1. Field of the Invention
The present invention relates generally to a MOS-type image sensor, and particularly to a MOS-type image sensor configured to reduce the influence of coupling noise between signal wires during operation. The present application is based on Japanese Patent Application No. 0387134/2000, which is incorporated herein by reference.
2. Description of the Related Art
A MOS-type image sensor may be manufactured using a standard MOS manufacturing process, which is different from a CCD image sensor, for example, which requires a special manufacturing process. MOS-type image sensors have recently received attention due to properties such as low power requirements as a result of low-voltage/single-power-source operation and ease of single chip fabrication for peripheral logic and macro circuits.
FIG. 1 illustrates a schematic diagram of a pixel portion for a MOS-type image sensor of the related art. The term pixel denotes a minimum unit and represent a point on a screen display, which is also called a picture element. In FIG. 1, the pixel portion includes a photodiode 31 that photoelectrically converts a received light input into an electric signal, a reset transistor 32 for initializing a potential at a cathode part of the photodiode 31, a transistor 33 for amplifying a signal photoelectrically converted by the photodiode 31, and a line selection transistor 34 for reading out the signal amplified by the transistor 33.
FIG. 2 illustrates a readout timing waveform chart for the MOS-type image sensor shown in FIG. 1. The signal-reset line 41 in FIG. 1 is activated to initialize the cathode part of the photodiode 31 and the pixel begins its exposure cycle. After a lapse of an arbitrary exposure time, the line-selection line 42 shown in FIG. 1 is activated and a signal that is photoelectrically converted in response to the amount of a light exposure is read out. While the time the line-selection line 42 is activated, the signal-reset line 41 is also temporarily activated. The line-selection line 42 remains activated during the activation of the signal-reset line 41. Hence, an initialized signal in the cathode part of the photodiode is successively readout. This operation is used for signal correction based on a reference signal inherent in each pixel.
The above-described operation is repeated for all pixels of a display in the order of the scanning direction, for example, from the top line to the lowest line, so that a picture signal for one frame is readout. In addition, upon reaching the lowest line, the scanning may again be successively repeated from the top line. Thus, it is possible to obtain a moving picture signal for a plurality of frames. In this case, for an arbitrary line, it is also possible to obtain an electronic shutter effect by activating only the signal-reset line 41 temporarily in an arbitrary time period from readout of a present frame to readout of the next frame. Thus, it is possible to obtain an electronic shutter effect that makes exposure time electronically variable.
FIG. 3 illustrates a related art example of the layout of a pixel portion in a MOS-type image sensor proposed by E. R. Fossum et al. (E. R. Fossum, SPIE, 1900, p 2-14, 1993). In this example, a signal-reset line R and a line-selection line S are disposed adjacent to each other at one end of a photoelectric conversion portion PG.
In the example shown in FIG. 3, the signal-reset line R and the line-selection line S are disposed adjacent to each other and both the signal-reset line R and the line-selection line S are made from the same wire material and are formed of wires with substantially the same width and thickness. As a result, as shown in FIG. 2, both the signal-reset line R and the line-selection line S are subjected to coupling noise 43 arising from capacitive effects between the wires during the rising and falling edges of each signal. For example, a problem arises if a signal photoelectrically converted in the photoelectric conversion portion PG becomes corrupted when the signal-reset line R malfunctions due to coupling noise 43 during activation of the line-selection line S.
The present invention solves the above-mentioned problems and is intended to provide a MOS-type image sensor configured to prevent a photoelectrically-converted signal from being corrupted during readout operation.